Modelling Ash Deposition during the Combustion of Low Grade Fuels

Abstract

Deposit formation on external surfaces, such as heat exchangers, is a serious problem in many combustion processes. Deposits form from residue ash particles and from volatilized species that deposit either as tiny condensed particles or as vapour. With modelling it is possible to make predictions about the rate and the chemical composition of deposition resulting from a specified fuel fired in different process conditions. This paper discusses the modelling of different deposit formation mechanisms that are used in the ABC (Aerosol Behaviour in Combustion) code (Jokiniemi et al., 1994). When modelling deposition it is essential to know the ash particle size distribution and chemical composition and well as the composition of the flue gases. The emphasis in the ABC code and in this paper is in the deposition of species that have volatilized during combustion. The chemical composition of these species is often such that in deposits they make surfaces sticky for residue ash particles or their presence may lead to the sintering of the deposited layer or their presence can cause high temperature corrosion. The ABC model couples the description of deposition mechanisms with the description of ash particle formation from volatilized species and the respective gas phase chemistry. The amount of residue ash particles is given in input for the ABC code. Detailed descriptions of the deposition models applicable to the process conditions are given in this paper. As example cases we present model results of deposition in a kraft recovery boiler and in a medium speed diesel engine.